Endless belts, which are utilized as circulating elements, are used in production systems to transport workpieces. For transport of such workpieces in a substantially horizontal direction, the workpieces can lie loosely on an endless belt or can lie loosely on a plurality of endless belts which are arranged side by side. For transport of such workpieces in any direction, they can be clamped between cooperating circulating endless belts that are arranged opposite one another. In either of these configurations, and in order to accurately convey the workpieces, it is important for the endless belts to each have a predetermined degree of tension. Such tension is needed either to limit the sagging of the belts, under the weight of the workpieces, to a predetermined level, or to ensure an adequately secure clamping of the workpieces between the cooperating belts which are arranged opposite one another.
A tensioning mechanism, that can be used to implement a desired tension to a circulating endless belt, generally comprises a main body, which is stationary in relation to a frame that supports the belt, and a head piece, such as a pulley, which is in contact with the belt. The pulley can typically be displaced, in relation to the main body, for the purpose of varying the path length of the belt, thereby varying its tension.
The technique of using a threaded spindle, for the purpose of displacing the pulley, is generally known. The rotation of such a threaded spindle causes the pulley to be displaced in relation to the main body. However, adjusting the belt tension using such a threaded spindle is laborious and time-consuming. Each time the pulley is adjusted, the belt tension must be measured to determine whether it is correct and to thus determine if further displacement of the pulley in one or the other direction is necessary. If a conveyor belt device has a plurality of circulating endless belts which are arranged in parallel, this belt tensioning process must be performed separately for each endless belt. Moreover, in an assembly comprising a plurality of tensioning mechanisms for use in the tensioning of a plurality of belts, it is difficult to transfer the adjusting motion to the threaded spindle of each individual adjustment mechanism within a compact structure.
The tensioning process can be simplified by the use of a tensioning mechanism that is actuated using a pressurized fluid. The pressure of the pressurized fluid applied to the tensioning mechanism determines the belt tension to be achieved rather than a position of the movable pulley. In other words, the application of the same pressure of the pressurized fluid to a tensioning mechanism can result in different positions for the pulley, depending upon the length and the elasticity of the endless belt to be tensioned. The requirement of measuring belt tension is thus eliminated. Moreover, a plurality of tensioning mechanisms, for use with a plurality of belts having the same transport direction, can be acted upon by pressurized fluid in parallel, in order to generate the same belt tension in all of the belts, regardless of potential variations in their lengths and/or elasticity.
However, it has been found that such a pressurized fluid or pneumatically driven belt tensioning mechanism produces greater wear and tear on the endless belts than does a mechanical tensioning mechanism.
DE 35 31 552 A1 discloses a tensionable belt reversal unit for a conveyor belt used in mining and tunnel construction. The tensionable belt reversal unit can be moved and locked in position via an actuator cylinder.
A chain conveyor for underground mining is known from DE 83 17 386 U1. A hoisting cylinder and a locking device are provided.
DE 18 01 522 A describes a tensioning mechanism for endless chain or belt mechanisms. This mechanism is provided with a pressurized medium cylinder.
DE 38 35 524 A1 discloses a reversing station for a scraper chain conveyor. A tensioning mechanism for the scraper chain belt is provided.
DE 25 54 785 A1, DE 100 14 700 A1, DE 43 15 504 A1, DE 42 30 781 C2, DE 35 40 880 A1 and DE 90 05 350 U1 all describe pressure actuated drives.
DE 31 13 894 A1 describes a pressure actuated operating cylinder. A piston rod is steplessly fixed in position via a pressure actuated, stationary device.
EP 0 890 532 A1 relates to a device for tensioning systems for transporting flat products in a folding unit. The device for transporting the flat products includes a plurality of individual belts.